1. Field of the Invention
This invention relates to a device for forming a thin film on a semiconductor substrate by the vapor growth method using plasma, and particularly it relates to a semiconductor processing device that is characterized by the shape of a showerhead and/or a susceptor.
2. Description of the Related Art
FIG. 1 outlines a conventional parallel-flat-plate type plasma CVD film-forming device. The conventional plasma CVD film-forming device comprises a vacuum chamber 1, a showerhead 2 positioned upright within the vacuum chamber substantially horizontally and a susceptor 3 positioned substantially in parallel and facing the showerhead within the vacuum chamber 1.
In the vacuum chamber 1, an exhaust port 5 leading to a vacuum pump (not shown) is used for vacuum exhausting the inside of the chamber.
At the base of the showerhead 2, multiple fine holes 11 for emitting a jet of material gas are positioned. The showerhead 2 is also linked to a material gas supply tank 6 through a line 10. On the line 10, a mass flow controller 8 for controlling a flow of the material gas is positioned. An RF power source 4 is also electrically connected to the showerhead 2, and functions as one side of the electrodes.
The susceptor 3 is normally an aluminum column within which a heater 14 is embedded. The susceptor 3 is supported by a support 12 and can be also rotated, for example, by a rotating mechanism. The susceptor 3 is also connected to ground 13 and functions as the other electrode. On the surface of the susceptor 3, a semiconductor substrate 9 is loaded and is fixed by vacuum fastening, etc.
Operation of the conventional plasma CVD film-forming device is explained below.
First, gas within the chamber 1 is vacuum exhausted by the vacuum pump from the exhaust port 5, and preferably a low pressure is maintained within the chamber 1.
Next, a preselected material gas flowing from the material gas supply tank 6 is controlled by the mass flow controller 8 at a preferable flow. A material gas controlled at a preferable flow is transported to the showerhead 2 through the line 10 and is jetted out from the multiple fine holes 11 provided at the base, toward the semiconductor substrate.
After a flow is stabilized, a radio-frequency (RF) electric field is generated between the showerhead connected to the RF power source and the susceptor 3 grounded to the earth 13. The above-mentioned material gas within the chamber 1 is ionized and a plasma state occurs. Atoms of the ionized material gas show a chemical reaction at a reaction region on the semiconductor substrate, and a desirable thin film is formed on the semiconductor substrate.
As a material gas, silicon source gasses such as SiH4, DM-DMOS[(CH3)2Si(OCH3)2] and TEOS, fluorine source gasses such as C2F6, oxidizing gasses such as oxygen and inert gasses such as Ar or He can be used.
The type and quality of a film formed on the surface of the semiconductor substrate 9 change according to the type, flow and temperature of the material gas, the RF frequency type, and the plasma""s spatial evenness.
The evenness of a film formed on the semiconductor substrate and the evenness of plasma density at the reaction region are closely related. As shown in FIG. 1, a distance between the susceptor 3 and the showerhead 2, i.e., a distance between the semiconductor substrate 9 and the showerhead 2, is fixed for a conventional plasma CVD film-forming device. In general, in the parallel-flat-plate type plasma CVD film-forming device, an electric field intensity distribution generated between two plane electrodes (Ø250 mm) has the property of being strongest at the center and gradually weakening toward the outer edge along a radius. In the film-forming region of a semiconductor substrate of Ø200 mm, the intensity distribution is approximately xc2x15%. Consequently, the electric field around the center of the semiconductor substrate 9 is relatively stronger than the electric field toward the outer edge along the radius, and the plasma density is also higher and the reaction of a material gas becomes more active. As a result, a thin film formed becomes thicker at the center, and the film quality becomes uneven at the center and at the outer area of the center.
This problem conventionally has been dealt with by controlling the flow or mixing ratio of gas supplied, the value of RF frequency applied and RF power energy. However, when these parameters are changed, the quality of the generated film and the film-forming speed change and stability of the process deteriorates. Particularly, if the mixing ratio and the flow of a material gas considerably affect the film quality, this problem becomes more serious.
It is important to resolve the problem of the evenness of a film due to the need for a larger diameter for semiconductor substrates in the future.
Consequently, an object of this invention is to provide a plasma CVD film-forming device that forms a thin film with an even film quality and an even film thickness on a semiconductor substrate.
Other object of this invention is to provide a plasma CVD film-forming device that forms a thin film with an even film quality and thickness for a substrate with a diameter of more than 300 mm.
Another object of this invention is further to provide a plasma CVD film-forming device at a low manufacturing cost and with a simple configuration.
To accomplish the above-mentioned objects, a plasma CVD film-forming device according to this invention comprises the following means:
A plasma CVD film-forming device for forming a thin film on a substrate, comprises: (a) a vacuum chamber; (b) a showerhead positioned within said vacuum chamber; and (c) a susceptor positioned substantially in parallel to and facing said showerhead within said vacuum chamber and on which said substrate is loaded, wherein the showerhead and the susceptor are used as electrodes and have surfaces facing each other, at least one of which surfaces is concave.
In the above, in an embodiment, the concave surface is a rotatably symmetrical surface around an axis of the showerhead or the susceptor.
In another embodiment, a distance between said showerhead and said susceptor satisfies the following relation:
fd=|dcxe2x88x92da|/daxc3x97100 
fd=1%xcx9c100% 
wherein:
fd is a deformation ratio of the central part of said showerhead""s surface that faces said substrate,
da is the average distance between said showerhead and said susceptor at an outer perimeter position of said substrate,
dc is the average distance between said showerhead and said, susceptor at a point on a radius of a distance equivalent to da from the center of said substrate.
Further, in yet another embodiment, a distance between said showerhead and said susceptor satisfies the following relation:
fdxe2x80x2=|dcxe2x80x2xe2x88x92daxe2x80x2|/daxe2x80x2xc3x97100 
fdxe2x80x2=1%xcx9c100% 
wherein:
fdxe2x80x2 is a deformation ratio of the central part of said susceptor""s surface that faces said substrate,
daxe2x80x2 is the average distance between said showerhead and said susceptor at an outer perimeter position of said substrate,
dcxe2x80x2 is the average distance between said showerhead and said susceptor at a point on a radius of a distance equivalent to daxe2x80x2 from the center of said substrate.
In an embodiment, a distance between the showerhead and the susceptor becomes greater toward the center and it becomes greatest at the center.
In the above, deformation ratios fd and fdxe2x80x2 can range from 1%xcx9c100% independently or concurrently. In an embodiment, deformation ratio fd or fdxe2x80x2 is 5-35%.
Deformation ratio fd or fdxe2x80x2 may be determined to render substantially uniform a distribution of electric field intensity over the substrate while forming a film thereon.
In the above, distance da or daxe2x80x2 may be in the range of 3 to 300 mm, preferably 5 to 100 mm. Difference |dcxe2x88x92da| or |dcxe2x80x2xe2x88x92daxe2x80x2| may be in the range of 0.3 to 50 mm, preferably 0.5 to 20 mm.
Deformation ratio fd or fdxe2x80x2 can be indicative of uniformity of quality and thickness of a film formed on a substrate. Additionally, a distance, dw, between the susceptor and the substrate can be indicative of quality and uniformity of thickness of a film and may be in the range of 0.1 to 10 mm, preferably 0.1 to 5 mm.
In an embodiment, the susceptor can have a diameter sufficient to support a substrate having a diameter of 300 mm or larger. A film can be formed on a large substrate.
Other conditions for processing a substrate can be the same as those conventionally employed. The showerhead may supply a material gas containing a compound selected from the group consisting of compounds which can be expressed by SixOyCzN1Hm wherein x, y, z, 1, and m are independently zero or an integer, including SiH4, Si(OC2H5)4, (CH3)2Si(OCH3)2, and C6H6. To the material gas, an additive gas such as He and O2 may be added in an embodiment. A radiofrequency power may be applied between the showerhead and the susceptor. Further, the susceptor may comprise a heater.
The present invention also relates to a method for forming a thin film on a substrate by using the aforesaid plasma CVD film-forming device. The method may comprise: (I) loading a substrate on the susceptor; (ii) controlling the atmosphere in the vacuum chamber; (iii) applying energy between the showerhead and the susceptor; and (iv) forming a thin film on the substrate.